Retrieving HTTP headers can be useful for inspecting metadata without downloading an entire resource.
With Java's URL and HttpURLConnection
classes, we can submit an HTTP HEAD request without too much effort:
private static Map<String, List<String>> getHeaders(URL url)
throws IOException {
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("HEAD");
connection.setDoInput(true);
connection.setDoOutput(false);
Map<String, List<String>> headers = connection.getHeaderFields();
connection.disconnect();
return headers;
}Each header can have multiple values, so headers are returned as a
key -> [value] map:
String url = "http://dumps.wikimedia.org/other/wikidata/20150209.json.gz";
Map<String, List<String>> headers = getHeaders(new URL(url));
String type = headers.get("Content-Type").get(0); // application/octet-stream
String size = headers.get("Content-Length").get(0); // 4071269760